Magnesium base alloys



Patented Mar. 22, 1949 UNITED STATES OFFICE MAGNESIUM BASE ALLOYS aBritish company No Drawing. Application March 8, 1946, Serial No.653,145. In Great Britain March 22, 1945 1 Claim.

This invention relates to the magnesium base alloys of the kinddescribed in the specification of British Patent No. 511,137 whichcontain zirconium but contain no additions of elements, such asaluminium, silicon, tin, manganese, cobalt, nickel, and antimony, whichin a molten magnesium-zirconium alloy combine with the zirconium presentto form high melting compounds and which may contain permissibleelements which are incapable of combining with the zirl conium dissolvedin the molten magnesium, to form high melting compounds which settleout.

We have now ascertained that special advantages are obtained by theinclusion in alloys of the foregoing kind of barium or strontium or bothbarium and strontium.

The total amount of these two elements barium and/or strontium may be upto 1% but as a matter of fact, traces of these elements are usuallysufiicient. The amount of these elements comprising said traces may bemerely such as are sufficient to produce a visible line when the alloyis spectroscopically examined by an uncontrolled condensed spark. Thequantitative lower limit for the beneficial action of these two elementsshould therefore be set at the order of 0.001%, but for practicalpurposes it is advantageous to have said element or elements present insomewhat larger amounts of the order of 0.01% to 0.1%.

It has been discovered that the above mentioned metals, barium andstrontium, exercise an entirely unexpected beneficial efiect whenalloyed together with magnesium and zirconium in that they aid theintroduction of zirconium into magnesium and magnesium base alloys,thereby refining the grain and notably increasing the proof stress, bothof cast and wrought alloys.

As is well known, it is difficult to introduce zirconium into magnesiumin such a state that it will eifectively act as a grain refiner andimprove the mechanical properties of the alloy. A great many methods foralloying zirconium with magnesium have been proposed, and many of thesealloy a greater or less proportion of the zirconium in a form in whichit is effective as a grain refiner. Whereas zirconium which is notefiective in refining the grain and improving the mechanical propertiesis found to be suspended in an insoluble state in the magnesium ormagnesium base alloy, the zirconium which is eiTective as a grainrefiner appears to be in solution and may be easily distinguished fromthe aforementioned insoluble zirconium because it is, contrary to anyother known form of zirconium metal, easily soluble in an aqueoussolution of hydrochloric acid consisting of 15 cos. HCl of sp. gr. 1.16to ccs. of water, sufficient acid being added during dissolution tomaintain the initial acid concentration.

Thus the term "insoluble zirconium used herein is intended to denotezirconium which is ineffective as a grain refiner, whilst the termsoluble zirconium used herein is intended to denote zirconium which iseffective as a grain refiner.

In the absence of barium or strontium the greatest grain refinementwhich can be obtained by the addition of zirconium to magnesium to forman alloy containing zirconium in the soluble state, which is die castinto a one inch diameter bar, results in an average grain size between0.045 and 0.060 mm.; but in the presence of about 0.05% of barium anaverage grain size between 0.025 and 0.040 mm. is normally produced whenthe resulting alloy is similarly die cast into a one inch diameter bar.However on occasion, grain sizes of the order of 0.015 mm. have been obtained, in the presence of barium. As is well known, mechanicalproperties are largely dependent on grain size, and this very fine grainsize is therefore highly desirable.

The alloys in accordance With the present invention should contain atleast 0.3% of soluble zirconium and they may in addition contain one ormore permissible elements such as zinc, cadmium, silver, cerium or otherrare earth metal, lithium or calcium.

Similarly, the proof stress of an alloy such as a magnesium basezinc-zirconium alloy is markedly influenced by barium and strontium bothin the as cast and the wrought state, as the following examples willshow:

Example I Three alloys A, B and C were made up containing the followingpercentages by weight of zinc, soluble zirconium, barium and strontium:

3 at a speed of 12 ins. per minute with the following resultant proofstresses:

0.1 Proof stress Alloy tons/sq. in.

Ewample II Two alloys, D" and E, were again made up to contain the fourabove mentioned elements in the following percentage proportions:

These alloys were again cast by the direct chill process and the proofstress of "the resulting cast metal determined.

% 0.1 Proof Alloy stress tons/sq. in.

Eazample III The above two alloys D and were extrudecl at-a temperatureof 370 C. at a speed of 15 ins. per minute with the following resultantproof stresses:

0.1 Proof Alloy stress tons/sq. in.

Example IV The above two alloys D and E were again extruded at atemperature of 376 C., alloy D ata speed of 4% ft. per minute and alloyE at a speed of 6 ft. per minute.

0.1 Proof Alloy stress tons/sq. in.

Example V It will be seen from Example IV that although the extrusionspeed of alloy D was slower than that of alloy the proof stress of alloyE was still greatly superior to that of alloy D. Even when alloy E wasextruded at a temperature of 370 C. at a speed of 13 it. per minute, 5the proof stress remained greatly superior to that of alloy D whenextruded at a speed of 1% ft. per minute.

0.1 Proof Alloy stress tons/sq.in.

In general, this increase of proof stress is accompanied by acorresponding decrease in elongation, and the elongation materiallydecreases when the quantity of barium or strontium exceeds about 0.1%.Nevertheless, the elongation of alloys containing about 0.1% of bariumor strontium still remains appreciable and sufilclent for many purposes,while the elongation of alloys in which the barium and/or strontiumcontent does not exceed 0.07-0.8% still remains very substantial. Thusthe elongations of the various alloys in the above examples were asfollows:

Elongation,

Example A B C D E D E D E E The elements barium and strontium may beadded to the molten magnesium in the form of the individual metals, orin the form of master alloys, or in the form of their fluorides oroxides. If used in the metallic form, the alloying is prefer'ablyeffected with the use of fluxes not containing magnesium halides, orunder an inert atmosphere such for instance as argon, 01' in vacuo.

A suitable method for introducing the zirconium, strontium and bariuminto the mag nesium is described in the specification of our co-pendingBritish application No. 7227/1945, according to which molten magnesiumis reacted with a fluid bath comprising a mixture of zirconium fluoride,and fluorides of barium or-strontium, and if desired or" calcium,lithium, and magnesium.

The inclusion of the elements barium and strontium and also lithium, ifthis element is present, appears to increase the tendency of theresultant alloys to oxidise, thereby renderingth'e usual inhibitors,such as sulphur and sulphur dioxide, inadequate when the metal is pouredat temperatures above 740 C. In the same way these alloys show aslightly increased tendency to attack green sand when the usualinhibitors are used in the usual proportions. However, if simpleprecautions are taken, no diiilculties in casting are experienced. Forexample, if a mixture'oi 1 part by weight of sulphur and 1 part byweight of boric acid, dried at 0., is shaken on to the pouring jet, andif a somewhat larger quantity than usual of sulphur and boric-acidisincorporated in the moulding sand, no undue oxidation results. It hasbeen found that 6 .per cent of sulphur and 4 per cent of boric acidincorporated in the moulding sand is satisfactory.

It is generally known by those skilled .in the art that magnesium basealloys containing zirconium are unusually diificult to prepare in acondition in which. they are free from contamination with flux and fromchloride inclusions, and so far all attempts to prepare uncontaminatedalloys have failed.

We have however made the surprising discovery that even minuteproportions of the two above mentioned elements, barium and strontium,have a remarkable and unforeseen effect on the elimination of fiux, inthat they considerably assist in the separation of the flux and ofchloride in-; clusions from the molten metal, not only during theinitial preparation, but also on remelting; REFERENCES CITED and as thepreparation of ingot for subsequent remelting is a most important outletfor mag- The following references are of record in the nesium basealloys, the property of remelting file of this patent: without fluxcontamination is of equal impor- TED ST TES T v tance to that of theinitial preparation of the UNI A PA ENTS metal in a flux free state.Number e Date We claim: ,8 0,1 4 Cooper Apr. 7, 1931 A magnesium basealloy consisting essentially 7 15 Fischer Mar. 9, 1937 of magnesium andat least 0.3 per cent of zir- 10 2,223,731 Sauerwald a 41 coniumeffective as a grain refiner, together with FOREIGN PATENTS at least onemetal from the group consisting of barium and strontium in total withinthe limits of Number Country Date 0,001 and Q1 per cent 511,137 GreatBritain Aug. 9, 1939 CHARLES J. P. BALL. 15

ALFRED CLAUDE J ESSUP.

